1. Field of the Invention
The invention relates to a control device for a vehicular automatic transmission and, more particularly, to a device for controlling an automatic transmission on the basis of a temperature of hydraulic fluid contained therein.
2. Description of the Related Art
As an automatic transmission to be installed in a vehicle, there has generally been employed a transmission that is constructed by combining a torque converter composed of a pump, a turbine, a stator, and the like with a multistage-type gear-shift mechanism connected to the turbine of the torque converter. This automatic transmission is usually equipped with a hydraulic circuit portion and a hydraulic control device as a main component. The hydraulic control device couples and releases hydraulically operated frictional coupling elements in the gear-shift mechanism such as clutches and brakes whereby a gear-shift operation of the automatic transmission is performed. In an automatic transmission equipped with such a hydraulic control device, when a gear-shift operation thereof is performed, hydraulic fluid pressure is supplied to a hydraulic circuit portion for operating frictional coupling elements. For establishing the hydraulic fluid pressure, an open period, an opening, or the like of a hydraulic control valve included in the hydraulic circuit portion is changed in accordance with a pulse duty factor or the like of a drive pulse signal supplied from a control unit to the hydraulic control valve. The hydraulic fluid pressure is thereby controlled. If the frictional coupling elements abruptly shift from a released state to a coupled state during control, a gear-shift shock may be caused. Accordingly, the hydraulic pressure supplied to the frictional coupling elements is regulated whereby the frictional coupling elements shift from a released state to a half-coupled state and then to a coupled state. In this manner, the frictional coupling elements can be shifted from an engaged state to a coupled state without causing a gear-shift shock.
As described above, in the case where the hydraulic fluid pressure supplied to the frictional coupling elements is changed in accordance with a predetermined characteristic, if the hydraulic fluid is at a low temperature, the viscosity thereof is higher in comparison with the case where the hydraulic fluid is at a high temperature, and a gear-shift operation in the automatic transmission undergoes a decline in responsiveness. Hence, although the frictional coupling elements are coupled at a suitable timing when hydraulic fluid is at a relatively high temperature, hydraulic fluid is supplied to or discharged from the frictional coupling elements with delay if the hydraulic fluid reaches a relatively low temperature. For this reason, there are some cases where the frictional coupling elements cannot be released or coupled at a suitable timing. As a result, a gear-shift shock may be caused.
Japanese Patent Application Laid-Open No. 64-35154 discloses a hydraulic control device for an automatic transmission as a solution to such a problem. The control device disclosed in this publication includes frictional engagement elements, a hydraulic control circuit, an oil temperature detecting circuit, and a control characteristic changing circuit. The frictional engagement elements are hydraulically operated and are designed to perform a gear-shift operation in the automatic transmission. To cause the frictional engagement elements to perform the gear-shift operation, the hydraulic control circuit supplies hydraulic fluid in accordance with a predetermined control characteristic. The oil temperature detecting circuit detects a hydraulic fluid temperature that generates a hydraulic fluid pressure. The control characteristic changing circuit causes the hydraulic control circuit to change a control characteristic during control of the supply of hydraulic fluid pressure to the frictional engagement elements in accordance with a hydraulic fluid temperature detected by the oil temperature detecting circuit.
According to this control device, the hydraulic fluid pressure supplied to the frictional engagement elements is changed on the basis of a control characteristic corresponding to the temperature of the hydraulic fluid contained in the automatic transmission Hence, the frictional engagement elements are operated without delay, for example, when hydraulic fluid is at a low temperature, and timings for engaging the frictional engagement elements and the like are suitably controlled.
A sensor for detecting a temperature of the hydraulic fluid contained in an automatic transmission is usually installed close to an exit of a torque converter that generates a large amount of heat and that tends to be a high-temperature area, for example, due to restrictions imposed on locations where the sensor can be installed. Hence, the sensor does not directly measure a hydraulic fluid temperature in the vicinity of an actuator or a valve that actually performs gear-shift control. Furthermore, since the hydraulic circuit of the automatic transmission is structurally complicated, the hydraulic fluid temperature varies widely with location. Thus, there are some cases where an oil temperature detected by the sensor is greatly different from a hydraulic fluid temperature in the vicinity of the actuator or the valve that actually performs gear-shift control. Especially in a low-temperature area, the hydraulic circuit itself has cooled. Therefore, hydraulic fluid is cooled while circulating through the hydraulic circuit or hydraulic fluid that has cooled continues to stay where it is. As a result, oil temperatures in the vicinity of the actuator and the valve as operating regions become lower than the oil temperature detected by the sensor.
If the control device disclosed in the abovementioned publication is applied under such a circumstance, gear-shift control cannot be performed on the basis of oil temperatures in actual operating regions. Therefore, a gear-shift shock may be caused.
If hydraulic fluid contained in the automatic transmission is at a low temperature, the viscosity thereof rises. Thus, a deterioration in responsiveness based on a clutch-to-clutch operation using a direct pressure is caused. Hence, as a solution different from the one disclosed in the above-mentioned publication, there is a control device that performs control in such a manner as to prohibit a gear shift to a specific gear stage if the hydraulic fluid temperature falls below a predetermined threshold. In such a control device as well, since prohibition of a gear shift cannot be controlled on the basis of an oil temperature in an actual operating region, the following problem is caused. Even if a measured oil temperature is high and hydraulic fluid in the operating region is at a low temperature, the threshold needs to be set high in advance in order to prohibit a gear shift to a specific gear stage. In the case where the threshold is thus set, if a difference between the measured oil temperature and the oil temperature in the operating region is small. nullification of prohibition of the gear shift is retarded and a gear shift to a higher gear stage is made with delay. As a result, driveability and fuel consumption are adversely affected.